I think the document actually mentions an (alleged) SRX 210 (if it's 310, let me know - it's a typo).
So what's happening here is that it's very difficult to tightly align software releases with hardware releases - and in this case it's resulted in accidentally revealing a product not yet announced. More information on the products should be available very shortly.
A good way to keep up with official product announcements is to use the RSS feed for our press releases:
http://rss.juniper.net/p/subscribe
Anything you see mentioned before it's in those press releases is mythical - like unicorns! 🙂
-Keith